Review and Research on Cancer Treatment Volume 3, Issue 1 (2017) ISSN 2544-2147

Protein 2 (CK2) in carcinogenesis

Monika Janeczko [email protected], Katedra Biologii Molekularnej, Wydział Biotechnologii i Nauk o Środowisku, Katolicki Uniwersytet Lubelski Jana Pawła II, ul. Konstantynów 1 I, 20-708 Lublin Abstract: The human kinase CK2 plays an important role in the physiological functioning of an eukaryotic cell, which is related to the regulation of the cell cycle, transcription, the DNA synthesis and repair, the mammalian circadian cycle and . The activity of CK2 also involves the formation of pathological conditions in cells, which results, among others, with the involvement of the enzyme in the process of tumorigenesis. CK2 is a heterotetramer composed of two catalytic subunits such as CK2α and/or CK2α' and two regulatory subunits CK2β. It is significant that the subunits are functionally independent from the holoenzyme, showing the structural and functional diversity at the same time and other catalytic properties. The increased level of the enzyme activity was confirmed in all types of tumors and promotes tumor cell growth in several respects: stabilizes oncokinom, counteracts the effectiveness of anticancer drugs, promotes neovascularization and most importantly generates a wide spectrum of survival signals of cell. This is the reason of growing interest in the possibility of regulating its activity towards the design and synthesis of specific and potent inhibitors, what may result in obtaining specific antitumor drugs in the future. Keywords: CK2; cancer; apoptosis, signaling pathways; anticancer therapy

1. Introduction Phosphorylation is one of the most of the genome) is common. In recent years 122 intensive post-translational protein protein genes have been identified in modification processes. It is estimated that yeast cells, 540 in mice and 518 genes in the about 30% of the proteins in eukaryotic cells human genome, although according to undergo reversible phosphorylation that can estimates their number may be as high as 2000 alter their functions, interactions, activity, [6÷9]. localization, stability and affect to the key Kinase activity is precisely controlled and cellular regulation mechanisms such as cell abnormalities in their functioning caused by cycle, protein activity such as tumor i.a. mutations disturb the functioning of the suppressor, mammalian circadian rhythm and whole signaling networks, leading to apoptosis [1÷5]. generation pathological and disease states. On Due to the huge importance of protein the basis of the comparison of human phosphorylation in cells, the occurrence chromosomal maps with identified disease of protein kinases encoded by one of the loci, a direct contribution of 164 kinases to largest eukaryotic gene families (about 2% tumor formation was confirmed [10, 11]. 2. General characterization of II The main element of regulatory and form of a holoenzyme, a hetero-tetramer signaling networks based on protein phospho- of approximately 130 kDa, consisting of two rylation in the eukaryotic cell is the CK2 catalytic subunits α (42-44 kDa) and α '(38 protein kinase (casein kinase 2 or II). It is kDa) and two regulatory subunits β (26 kDa), a serine / threonine kinase that uses both ATP so it can exist in configu-rations α α'β2, α2β2 or and GTP as phosphate residue donors. It is α'2β2 (fig.1) [12]. present in human cells most commonly in the

α/α α/α ' ' α α' β β

Figure 1. CK2 kinases can function as monomeric kinases and in a tetrameric complex [own elaboration].

The CK2 catalytic subunit consists of two allowing free domain rotation upon binding characteristic domains: the smaller N-terminal of ATP and / or substrate. The phosphate and the larger C-terminal, which are connected donor binding site placed between the two by a single polypeptide chain (linker region), main domains, interacts with the enzyme via

31 Review and Research on Cancer Treatment Volume 3, Issue 1 (2017) ISSN 2544-2147 hydrogen bonds. The ATP binding site is subunit, CK2β also consists of two domains. characterized by the presence of five specific The N-terminal region represents a larger regions: three hydrophobic (adenine region, domain and has a α-helical character. This hydrophobic pocket I and II) and two domain is associated with a smaller β structure hydrophilic (ribose region and phosphate (domain II), that contains a characteristic zinc binding region) that can be 4used to bind ion group. The C-terminal loop (so-called tail) different chemical groups of inhibitors is directly involved in the holoenzyme competing with ATP [13÷15]. formation. Catalytic subunits are attached to The CK2β regulatory subunit in the regulatory subunits dimer and do not mammalian organisms is encoded by one gene interact with each other. Each of the α subunits and does not show sequential similarity to any interacts with two β subunits [12, 14, 16, 17]. other regulatory protein. Like the catalytic 3. Cellular location, substrates and physiological importance of CK2 CK2 protein kinase is ubiquitous, leaving out transcription factors (~90), pleiotropic and highly conserved enzyme in cytoskeletal and structural proteins (~14). cells. It is widely located in Metazoa: plants, Approximately 40 substrates of CK2 were also animals and humans. It has been also identified identified among viral proteins [22]. in primitive protozoa and in fungi Apart from the enzyme-substrate type representants [12, 18, 19]. connection, a number of CK2 interactions with In mammals, CK2 activity has been non-substrate proteins have been described, demonstrated in most tissues and almost in every which comprise one of the elements of CK2 cellular compartment, mainly in the nucleus, activity regulation. This regulation takes place by cytoplasm, cell membrane as well as the principle of interactions with the α (e.g. Pin1, mitochondrium membrane, mitochondrial APC, IRS-1, CKIP-1, PP2A, Grp94) or β subunit matrix, endoplasmic reticulum, cytoskeleton, (e.g. p21WAF1, p53, TNP-1, FGF-2) or both (e.g. centrosome, Golgi apparatus and ribosomes [12, Nopp140, eIF2β), the consequence of this is 19]. CK2 enzymatic activity in different cell modulation of activity the subunits themselves or compartments is variable and regulated in the whole holoenzyme [23]. response to a variety of signals and impulse Among the substrate proteins, both inhibitors associated with cell cycle progression or cellular (nucleic acid phosphoprotein Nopp140, stress [20]. Moreover, nu-merous studies confirm translation initiation factor eIF2β, tumor the existence of catalytic and regulatory subunits suppressor p53), and activators of CK2 (e.g. independently of each other. Each of them can HSP90 heat shock protein, nucleolin) were function separately, and their roles often remain identified. Interactions with proteins regulating different from the functions they perform in kinase activity may result in CK2 targeting to holoenzyme. Catalytic subunits preserve specific cellular structures, or modulating enzymatic activity and often is differ in substrate specificity for protein substrates (e.g. Pin1- specificity. There is evidence of the dynamic reducing specificity for topoisomerase IIα, FACT localization of individual subunits and the – enhancing specificity for p53). So far, the independent displacement of α and β within the contribution of 68 proteins to direct modification cell [12]. of CK2 activity has been confirmed [23÷25]. The presence of CK2 kinase in various CK2 is an important element of every cell cellular compartments is associated with the cycle step. In mammalian cells, its activity is phosphorylation of many substrates and crucial in the transition and progression G0/G1, consequently the involvement of the enzyme in G1/S and G2/M [26, 27]. Important elements regulation of important cellular processes such as of the cell cycle control, that remain under the differentiation, mobility, cytoskeletal control of CK2 kinase are among others: CAK reorganization, proliferation, RNA synthesis, kinase, p53 protein, SSRP1 proteins, FACT apoptosis and transformation [21]. elongation factor element, MDM-2, p21WAF1/CIP1, So far, hundreds of physiological kinase p27KIP1, β-tubuline, Cdc25B, tau protein, PP2A, substrates have been identified, and among them topoisomerase II, Chk1 kinase, CCdc34, Cdk1, transcription factors (~ 60 proteins), proteins Six1 and proteins associated with microtubules regulating the functions of nucleic acids and 1A and 1B [28]. The contribution of CK2 kinase protein synthesis (~50), signaling proteins to the degradation pathway has also been

32 Review and Research on Cancer Treatment Volume 3, Issue 1 (2017) ISSN 2544-2147 confirmed, and consequently its influence on the are regulated at the level of expression. One initiation of cell division [29]. of the proteins is survivin, belonging to the CK2 is an important element of transcription group of apoptosis inhibiting proteins known control. The effects of the kinase activity involve as IAPs (Inhibitory Apoptosis Proteins), multiple levels of regulation and a multitude activity of which is inhibited by CK2 of substrates. In particular, this regulation applies overexpression [33]. In turn, the to the basic elements of the transcription phosphorylation of the Bid protein, a pro- mechanism, i.e the RNAP I, RNAP II and RNAP apoptotic member of the Bcl protein family, III polymerases [28]. protects it against caspase-8 activity, inhibiting Regulation of activity by phosphorylation mitochondrial apoptosis mechanism [34]. This or on the basis of other interactions with CK2 type of regulation, where protein concerns also transcription factors: NFκB, phosphorylation prevents caspase activity, STAT1, CREB, IRF-1 and IRF-2, ATF1, SRF, indicates an evident antiapoptotic role of CK2. Max and protooncogenes: c-Jun, c-Fos, c-Myc This phenomenon is determined by a similar and c-Myb [28]. sequence, rich in aspartate residues, recognized Recently, the role of CK2 kinase in cellular as a caspase cleavage site and simultaneously processes that decide about cell entering the phosphorylated by CK2 [12]. A similar apoptosis pathway and the pro-life character regulatory mechanism applies also to other of the kinase is the subject of intensive caspase substrates: Max protein, HS1, research. Overexpression of CK2 protects presenilin-2, conexin 45,6 and PTEN [28]. against drug-induced apoptosis and vice versa, CK2 also regulates the activity of caspases kinase overexpression is often observed in cell themselves. It has been proven that the lines that are resistant to apoptosis-inducing caspase-9 phosphorylation in mice model drugs. It is an enzyme directly involved in both protects it against caspase-8 activity, as well as types of pro-grammed cell death: extrinsic and the inhibitory effect of CK2 on caspase-2 intrinsic, induced by DNA damage, and dimerization and thereby its inactivation has inhibition of its activity induces apoptosis in been shown. Furthermore, the ARC protein, tumor cells, which provides a promising aspect inhibiting caspase-8 activity, also remains in cancer therapy [30÷32]. under CK2 control [35÷37]. Many of the apoptosis signaling pathway proteins are direct substrates of CK2, others 4. CK2 contribution in tumor processes In addition to basic and key physiological kinase activity is observed in all known types functions, CK2 kinase is involved in the of tumors, including head and neck, kidney, generation of many diseases, including colon, lung, prostate and breast cancer neurodegenerative, viral, parasitic, inflammatory [38÷43]. conditions and in many types of cancer. High CK2 activity promotes tumor cell CK2 level in cells remain at a constant growth in a number of respects: a) improves characteristic level. It is relatively high in some transformation potential of oncogenes, b) organs such as in the brain or in the testicles, stabilizes oncokinom by activation of co- which represents a normal physiological chaperone CDC37, which is crucial for the condition. It rises during cell proliferation, maintenance of the active conformation whereupon it reaches a stable level that is of kinases with oncogenic potential, c) crucial for cell homeostasis. Instead, in tumor counteracts the effectiveness of anti-neoplastic cells elevated kinase activity is observed in the drugs, especially imatinib and melphalan, d) cell nuclei and deregulation of the kinase promotes neovascularization, and most activity is observed in the disease importantly e) generates a broad spectrum intensification states, and it even serves as of pro-life cell signals [44]. a prognostic indicator. Increased levels of the

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Table 1. Known mechanisms by which CK2 plays a global role as a pro-survival and anti-apoptotic agent CK2 Potentiates the Akt pathway Promotes I κ B degradation and activates NF-kB Stabilizes Dvl and β-catenin upregulating the Wnt pathway Generates caspase resistant sites in Max, Bid, HS1, PTEN, connexin 45, caspase 9 etc. Phosphorylates and activates the caspase inhibitor protein ARC Promotes rRNA and tRNA biogenesis Promotes the degradation of tumor suppressor PML Facilitates DNA repair Source: [44]

Many of these CK2 functions, particularly NF-κB is a involved in growth signals maintenance, apoptosis expression of cellular , D1, anti- inhibition, involvement in angiogenesis lead to apoptotic proteins (BcL-xI and IAP). It is usually the changes in cell physiology characteristic for located in the cytosol, where the interaction with carcinogenesis. an inhibitor IκB inhibits its activity. Degradation Table 1 presents known mechanisms in which of IκB in the SCF-B-TrCP proteasome pathway CK2 is involved, favoring the formation and releases the NF-κB factor, targeting it to the cell maintenance of a neoplastic cell phenotype. In nucleus. CK2 works on several stages of this addition to the basic functions described in the process. First and foremost, it activates IκB previous chapter, i.e. participation in growth and proteolysis, which constitutes an alternative proliferation regulation, rRNA and tRNA pathway, alongside the basic IKK kinase- biogenesis, DNA repair, caspase inactivation, dependent pathway, expression of which is also CK2 kinase also affects regulation of anti- under the control of CK2. Moreover, the p65 apoptotic proteins and pathways, i.e. NF-κB, subunit of the NF-κB factor undergoes PI3'K / Akt and Wnt (Fig. 2, 3 and 4) [44]. phosphorylation, which in turn increases its activity (fig. 2) [45, 46].

Figure 2. CK2-dependent multisite regulation of NF-κB. A negative effect (−) is indicated by a dot-arrow, and means inhibition or increased degradation, while a positive effect (+), indicated by a normal arrow, means enhanced stability and/or activity [44].

The plays proliferation by maintaining a high level of β- an important role in embryogenesis, while its catenin, which is a cofactor for a group of TCF activity in adult individuals promotes / LEF transcription factors involved in transformation and carcinogenesis [47]. CK2 is expression of pro-life signals: c-Myc, c-Jun involved in the reactivation of the pathway, and D1. Phosphorylation of β-catenin by which has been observed in case of colorectal CK2 is a key element of the stabilization cancer. The Wnt pathway regulates cell of this protein and of the protection against

34 Review and Research on Cancer Treatment Volume 3, Issue 1 (2017) ISSN 2544-2147 proteasome degradation. The reverse effect is phosphorylation of UBC3 and UBC3B induced by phosphorylation of β-catenin by the proteins, which interact with the F-box, GSKβ pathway kinase, which leads to an element of the B-TrCP proteasome complex ubiquitination and degradation of the protein. (fig. 3) [48, 49]. CK2 has also been shown to be involved in the

Figure 3. CK2-dependent multisite regulation of β-catenin. A negative effect (−) is indicated by a dot-arrow, and means inhibition or increased degradation, while a positive effect (+), indicated by a normal arrow, means enhanced stability and/or activity [44].

The contribution of CK2 in the stabilization and activation of pro-life signals take place and regulation of β-catenin level in the Wnt [48]. pathway also affects phosphorylation of Dvl The tumor suppressor – APC protein is proteins. These proteins are responsible for a negative regulator of Wnt signaling, GSKβ activity regulation by blocking the simultaneously interacting with CK2 via the α ability of β-catenin phosphorylation, which in subunit and as a consequence it inhibits the turn enables phosphorylation by CK2. As activity mainly of the holoenzyme of the a result, dissociation of the protein from APC kinase. This effect achieves the highest level in and Axin proteins, translocation to the nucleus G2 / M phase [40].

Figure 4. CK2-dependent multisite regulation of Akt. A negative effect (−) is indicated by a dot-arrow, and means inhibition or increased degradation, while a positive effect (+), indicated by a normal arrow, means enhanced stability and/or activity [44].

The effect of CK2 on cell survival is The progression of the pathway is inhibited by particularly visible in PI3'K / Akt pathway. PTEN phosphatase which dephosphorylates

35 Review and Research on Cancer Treatment Volume 3, Issue 1 (2017) ISSN 2544-2147 phosphatidyl inositol 3,4,5-triphosphate with Hsp90 that protects Akt against (PIP3), acting antagonistic towards PI3'K dephosphorylation [51÷53]. kinase. The CK2-mediated PTEN Another mechanism by which CK2 affects phosphorylation inactivates this enzyme, the activity of tumor suppressor proteins is which in turn stimulates Akt-dependent based on regulation of vulnerability to signaling. Interestingly, in most tumor cells proteasome degradation. This type PTEN activity is lost, whereas in T-ALL of regulation applies to i.a. PML protein primary cells a high level of PTEN is involved in the control of many pathways maintained compared to normal T- responsible for growth inhibition, apoptosis or lymphocytes precursors, which is cell ageing. The loss of PML activity is simultaneously associated with high CK2 observed in many cancers and correlates with expression. Thus, the constitutive activity the tumor progression. CK2 phospho-rylates of the PI3'K / Akt pathway is possible not only PML at Ser-517, which is critical for directing in the inhibition of PTEN expression, but also the protein to degradation, and consequently in the inhibition of phosphatase activity by protects cells against apoptosis [50]. high CK2 level [44, 47, 50]. The CK2 implication in signal cascades is The direct effect of CK2 on Akt activity often untypical in comparison with other was also demonstrated by Thr-308 kinases, primarily because it is not phosphorylation in the catalytic domain and a component of hierarchical dependence, Ser-473 in the C-terminal domain, as well as remains beyond the molecular regulatory Ser-129 phosphorylation, which generates mechanisms, simultanously integrates and a constitutive kinase activity. CK2 contributes consoli-dates the various connections and to maintaining a high level of Thr-308 pathways. Hence deregulation of CK2 activity phosphorylation, providing a stable connection fosters such profound and diversified changes in cell biology.

Figure 5. A schematic model for the role of CK2 in carcinogenesis. High levels of CK2 expression and activity have been illustrated in a variety of cancers [54].

Inhibition of CK2 activity by antisense RNAi, in order to silence DNA expression supports overexpression of inactive kinase form or therapy of rhabdomyosarcoma and colorectal chemical inhibition sensitizes cancer cells to cancer cells, making them sensitive to TNF- induction of apoptosis by chemo- or related apoptosis-inducing (TRAIL) radiotherapy. This dependence was confirmed [54÷55]. and used i.a. in chemical induction of apoptosis Undoubtedly CK2 plays a significant role in T-lymphoblastic cells or reactive oxygen as an antiapoptotic and pro-life factor at many species (ROS) dependent apoptosis in leukemia levels of its activity, as summarized in the cells. The application of small interference RNAs figure 5. 5. Inhibitors of CK2 CK2 kinase is involved in many fundamental pathological conditions. Hence the growing aspects of the physiological functioning of the interest in the possibility of regulation of its cell, on the other hand promotes conditions activity through the design and synthesis conducive to carcinogenesis and other of specific and strong inhibitors, which in the

36 Review and Research on Cancer Treatment Volume 3, Issue 1 (2017) ISSN 2544-2147 future may result in the acquisition of specific drugs. 5.1. ATP-competitive inhibitors The human genome encodes more than 500  halogenated benzimidazole and protein kinases, which are characterized by high benzotriazole derivatives, eg. DRB (5,6- conservative ATP binding site. However, there dichloro-1-β-D-furanosyl-benzimidazole), are some structural differences, mainly in TBB (4,5,6,7-tetrabromo-1H- proximally located regions, which generate the benzotriazole) and its 2-dimethylamino selectivity of ATP-competitive inhibitors. In in derivative (DMAT), TBI (4,5,6,7- vivo studies, inhibitors often show a limited tetrabromo-1H-benzi-midazole), TIBI membrane permeability and a poor physiological (4,5,6,7-tetraiodo-1H-benzimidazole); effect. It is not without significance, that the  pyrazole-thiazine derivatives (according to cells exhibit high levels of ATP (1-10 mM), PDB labeled 3BE9, 2PVH, 2PVJ, 2PVK, which is important primarily in inhibiting 2PVL, 2PVN); constitutively active kinases. In case of CK2,  fluorenone derivatives such as FL12 (2,7- the high affinity of the enzyme to ATP also dihydroxy-3,6-dinitro-fluoren-9-one) and remains problematic [56]. benzonaphthon derivatives eg. THN There are several groups of chemicals that (tetrahydroxy-benzonaphthon); exhibit differentiated efficacy and specificity  carboxylic acid derivatives, eg. IQA (5- towards CK2. These are, i.a. the compounds oxo-5,6-dihydroindole- (1,2-a) -quina- of natural origin: zolin-7-yl-acetic acid), tetrabromic  flavonoids, eg. apigenin, quercetin, cinnamic acid derivative (TBCA), myricetin and fisetin; tribromic benzoic acid derivative and  coumarins, eg. DBC (3,8-dibromo-7- tetraiodic propionic acid derivative hydroxy-4-methylchrom-2); TID46;  antrachinons and xanthenones, eg. emodin  3-carboxy-4-(1H)-quinolones, eg. 5,6,8- (1,3,8-trihydroxy-6-methyl-antra- trichloro-4-oxo-1,4-dihydroquinoline-3- quinone), 1,3,8-trihydroxy-4-nitro- carboxylic acid; anthraquinone (MNA), 1,8-dihydroxy-4-  antimonic acid derivatives, eg, (E)-3-(3- nitro-xanthan-9-one (MNX) and 1,4- antimonophenyl)-prop-2-enoic acid diamino-5,8-dihydroxyanthraquinone (IC50 = 0,15 μM); (DAA), quinalizarine (1,2,5,8-  xanthene derivatives with negatively tetrahydroxy anthraquinone); charged carboxyl or sulfonic groups, eg.  ellagic acid; 2,3,4,5-tetrabromo-6-6-hydroxy-3-oxo-  rezorufin [57÷59]. 3H-xanthen-9-yl-benzoic acid [57÷59]. A significant group of competitive Structures of selected inhibitors are shown inhibitors against ATP are synthetic in the figure 6. compounds:

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Figure 6. Chemical structures of selected ATP-competitive inhibitors of CK2 [own elaboration]

It is worth emphasizing that to this class and other key mediators such as p21 protein. of compounds belongs the first CK2 inhibitor Moreover, it selectively induces apoptosis in which has successfully passed phase I cancer cells and exhibits antiproliferative and of clinical trials, i.e. CX4945 (5-(3-chloro- anti-angiogenic effect. It is effective in the phenylamino)-benzo-naphthyridine-8-carboxylic treatment of solid tumors and multiple acid), also known as Sil-mitasertib. Activity myeloma, showing promising tests performed on more than 145 kinases have pharmacodynamic and pharmacokinetic confirmed its high selectivity for CK2. It properties. Currently CX-4945 is in phase I/II shows a wide spectrum of antiproliferative clinical trials in the United States, South activity on various cancer cell lines such as Korea, and Taiwan for the treatment lung, breast cancer cells and prostate cancer. It of cholangiocarcinoma in combination with has been demonstrated that the mechanism gemcitabine and cisplatin (NCT02128282). of antitumor activity of this compound is based The aim of this trial is to determine its on the inhibition of the processes in which maximum tolerable dose in patients followed CK2 kinase is involved, directly related to the by a randomized phase II assessment using maintenance of the tumor cell phenotype. This CX-4945 in combination with gemcitabine and is an inhibition of the PI3K / Akt pathway by cisplatin versus the standard of care [60÷63]. suppression of phosphorylation of Akt kinase

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5.2. Non-competitive inhibitors towards ATP An alternative to ATP-competitive Structures of these inhibitors are shown in inhibitors are compounds that do not compete the table 2. directly with ATP for active site, and inhibition CIGB-300 is a second CK2 inhibitor tested in of enzyme activity is based on a variety I/II Phase of clinical trials. In the clinical ground, of mechanisms using structural and functional this synthetic peptide has proved to be safe and characteristics specific to CK2. These well tolerated in a First-in-Human trial in women compounds generate less side effects and with cervical malignancies who also experienced higher specificity of action mainly because signs of clinical benefit. In a second Phase I clinical trial in women with cervical cancer stage of their targeting to less structurally IB2/II, the MTD and DLT have been also conservative CK2 regions. Due to the fact, that identified in the clinical setting. Interestingly, in they do not compete with cellular ATP, they cervical tumors the B23/nucleophosmin protein can be used in concentrations close to levels were significantly reduced after CIGB-300 biochemical Ki value. On the other hand, they treatment at the nucleus compartment [68]. It are characterized by limit inhibition power should be noted that CIGB-300 modulates resulting from low affinity and intracellular several CK2-dependent signaling pathways. In instability [64]. Examples of such inhibitors NSCLC models (non-small cell lung cancer), together with the mechanisms of their action CIGB-300 induced an anti-proliferative response. are listed below: This effect was accompanied by the inhibition  polyanionic inhibitors (eg. heparin and of the NF-κB pathway, which was associated other acid polysaccharides, eg. poly- with an enhanced proteasome activity. Moreover, the NF-κB pathway appeared to be critically glutamic acid and pseudo-substitutive involved in the cisplatin-resistance of A549- peptides) interacting with a substrate cispR cells, which became more sensitive to binding site rich in base residues [59]; CIGB-300 treatment [69]. Knowable data  CIGB-300 (phage cyclic P15 peptide suggest a potential use of CIGB-300 as a novel formed after fusion with the cell therapeutic agent against lung cancer, because penetrating Tat peptide), interacting with this peptide markedly decreased lung the phospho- site of CK2 colonization and metastasis development substrates, especially with B23 of murine 3LL cells in mouse models and oncogene/ nucleoplasmin [65]; significantly reduced tumor cell-driven  inorganic ionic transition metal neovascularization [70]. complexes, mainly tungsten, CK2 is a very interested candidate for targeted therapy, with two inhibitors in ongoing molybdenum and vanadium in the form clinical trials. CX-4945 is a bioavailable small- of oxoligands (POM). The mechanism molecule ATP-competitive inhibitor targeting its of action of these compounds is active site, and CIGB-300 is a cell-permeable connected with the impact on the key cyclic peptide that prevents phosphorylation structural elements of the enzyme, and of the E7 protein of HPV16 by CK2. In in this case, the activation site blocks preclinical models, either of these inhibitors CK2 in an inactive conformation [66]; exhibit antitumor efficacy. Furthermore, in  peptide (P1) that interacts with the N- combinations with chemo-therapeutics such as terminal domain of CK2β and blocks cisplatin or gemcitabine, either CX-4945 or interactions between CK2β and specific CIGB-300 promote synergistic induction ligands [67]. of apoptosis [71].

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Table 2. Non-competitive inhibitors with the mechanisms of their action. Type of Type of Examples interaction molecule POM Allosteric Inorganic inhibitors compounds

CK2β Peptide P1 binding Peptides inhibitors GKMNGVLPLAWPSLYLRL

Cyclic Peptide Pc peptide GCRLYGFKIHGCG

Inhibitors W16 inhibiting Organic O compounds O interactions O O CK2α/CK2β O O O N O O

Cyclic Peptide P15

Inhibitors peptide CWMSPRHLGTC

inhibiting the attachment of Hematein a protein OH Organic substrate OH compounds HO O

O OH

Source: own elaboration

6. Conclusion There is strong evidence that CK2 plays The ability of CK2 to promote tumors a role in the pathogenesis of cancer. CK2 is causes the CK2 has emerged as a potential overexpressed in many cancers and often anticancer target. The wide range of cell- overexpression is associated with worse permeable chemical CK2 inhibitors have been prognosis. CK2 is involved in many key developed. The most frequently used are TBB, aspects of cancer including inhibition quinalizarin, hematein, TBCA, CIGB-300, of apoptosis, modulation of signaling CX-4945, DRB, apigenin, DMAT, and pathways, DNA damage response, and cell emodin. Two of these CX-4945 and CIGB-300 cycle regulation. This enzyme has the ability to have made into preclinical and clinical trials. regulate pathways, which These inhibitors are already used in phase I/II may vary in different cancers, such as Wnt trials in certain malignancies like lung, head signaling, JAK/STAT, NF-κB, and and neck cancer, cholangiocarcinoma, cervical PTEN/PI3K/Akt-PKB. Furthermore CK2 can cancer and multiple myeloma with promising be used as a diagnostic and prognostic marker results for the future. in certain malignancies, such as prostate cancer.

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